Solid-state preparation of foamed lead



Jan. 31, 1967 N. A. RATCLIFF ET AL 3,301,672

MED LEAD United States PatentO" 3,301,672 SQLID-STATE PREPARATEON OF FOAMED LEAD Nigel Alec Ratcliif and David Henry Roberts, Harlow, England, assignors, by mesne assignments, to St. Joseph Lead Company, New York, N.Y., a corporation of New York Filed Aug. 1, 1963, Ser. No.-299,294 Claims priority, application Great Britain, Aug. 3, 1962, 29,988/ 62 Claims. (Cl. 75-201) This invention relates to lead and in particular to the production of a relatively low density lead.

In U.S.A. application No. 71,410, now Patent No. 3,098,293 and application No. 269,030 there are described methods of producing dispersion hardened or strengthened lead involving mechanical treatment of lead powder to fragmatise an oxide coating on the powder particles and to cause contiguous particles to become pressure welded together to form a coherent mass having the oxide fragments dispersed therein. The mechanical treatment ineludes extrusion of the coated lead particle's either in the as received condition or according to application No.

269,030 after a ball-milling or like treatment of the lead powder in order to increase the percentage content of oxide resulting from natural oxidation of the particles in air (application No. 269,060).

It has been found that if a body of lead so prepared is heated it shows a significant degree of permanent exating substance other than lead carbonate. Dependent on the amount of dispersed oxide (as distinct 'from carbonate) the strength of the cellular lead can be as great as or greater than" the strength of ordinary lead due to the dispersion strengthening effect of the dispersed oxide.

According to the invention, therefore, a method of producing low density lead comprises the steps of obtaininig' lead powder with the particles thereof coated with a substance which is' capable of liberating gas when heated to a temperatu're'below the melting point of lead, working thepowderfpreferably by extruding it, to deform the particles so that the coating substance breaks into frag ments and the contiguous particles become pressure welded together to form a coherent mass with said fragments dispersed therein, and heating said mass to a temperature below the melting point of lead but above that at which 7 gas is liberated from the coating substance. Prior to the heating step the Worked mass may be formed, as by rolling to a desired shape such as strip or sheet form.

The permanent expansion which results from the heating is demonstrated in a striking way in the accompanying drawing which shows a section through a block of lead made in accordance with the invention by extrusion of coated lead powder followed by heating to 320 C. for one hour. Before the block was heated a wire was bound round it at position A-A' to prevent it expanding there,

the lead particles were initially coated with a gas liber- 3,301,672 Patented Jan. 31, 1967 so that the dimension of the expanded block at this position represents the original dimension of the block.

In carrying out the invention the gas liberating substance is preferably lead carbonate as previously mentioned, in view of the relative ease by which the particles of lead powder can be caused to acquire a coating of or including this substance. For instance, they can be coated by simply exposing the particles to the action of air (producing a coating consisting of both lead oxide and lead carbonate)v or, if a greater percentage Weight of lead carbonate in the powder is wanted, by subjecting them to an atmosphere of carbon dioxide and water vapor. To obtain a greater percentage of both oxide and carbonate (the former to increase the strength of the final product) the lead powder may be subjected to the action of both oxygen and carbon dioxide/water vapor.

The working of the lead to break up the coating substance and form the coherent mass with the coating fragments dispersed therein may consist simply of extrusion of the coated powder, but this may be preceded if desired by a ball-milling or other such treatment to increase the initial oxide content. The extrusion may be effected with or without heating. Cold extrusion is suitable with low extrusion ratios but with higher extrusion ratios some heating during extrusion may be desirable in order to .facilitatethe extrusion: however this heating must not be such as to melt the lead or even to cause premature liberation of gas. Providedthat lead carbonate or other suitableygas liberating substance can be obtained in sufficiently finely divided form a similar result may be achieved simply by taking a powder comprising a mixture of particles of this substance and of lead, Working this powder so that contiguous lead particles become pressure welded together to form a coherent mass with the particles of the gas liberating substance dispersed therein, and thereafter heating the mass as before. It is contemplated however that best results will be obtained using coated lead rp 1 Low density cellular lead produced in accordance with the-invention can be used for instance for sound attenuation panelling, for heat, insulation, for radiation shielding, for space filling, for sealing and so on, it being anticipated that in appropriate circumstances a pellet of the worked but not. yet expanded lead could be introduced into a cavity to be filled and then heated as by induction heating to effect the gas liberation and increase the bulk of the lead mass to fill the cavity.

Table 1 below shows as percentages the increases in volume which have been obtained with lead produced in accordance with the invention, starting with lead powder prepared by atomization of liquid lead. Separate batches of this powder, the particles of which had acquired by the atomization a-natural oxide coating containing some basic lead carbonate, were subjected to an atmosphere of carbon dioxide and water vapor-blown through 'the powder so as to fluidize it and thereby ensure that all particles acquired a thicker coating of lead carbonate. The carbon dioxide/ water vapor atmosphere was obtained by bubbling carbon dioxide through water. The several batches of the powder were subjected to this carbonating treatment for different times and a further batch was taken untreated, each batch therefore having a dilferent carbonate content. The carbonate percentages by Weight for the different batches are shown in the table. Each batch of powder was extruded at room temperature (25 C.) through a die of 0.450 inch diameter, the extrusion ratio being 20:1. The powder was then extruded and samples from each batch were heated to diiferent temperatures for difierent times and the increase in volume of the final products (with corresponding decrease in density) were determined. The results obtained are shown in the table.

TABLE 1 Heating Percent Expansion with carbonate eon- Heating Tern- Time tent perature 0.) (hours) 150 14 0.23 0.84 1.42 1.70 1 0.30 0.51 1.03 2.15 2 0.26 1.10 2.81 3.08 4 0.28 2 .26 3.43 4.19 6 0.28 3 .16 4.61 .08 8 0.39 4.09 5.20 6 .05 200 ta 0.39 8.70 10.98 16 .28 1 0.49 .01 13.53 18 .39 2 0.60 10 .98 16 .49 20 .04 4 0 .92 11 .98 17 .57 21 .28 6 1.18 12.07 21.25 21 .81 8 1.42 12 .90 21.46 23 .85 250 5i 2 .45 13.11 21.20 22 .41 1 3 .82 13.36 21.63 23 .62 2 6 .12 15.13 24.03 23 .78 4 9.70 16.44 25.00 24.60 6 11.45 18 .06 25 .02 25 .87 8 12.31 18.78 25 .63 29 .03 300 ,4 17 .44 22 .41 28 .61 29 .75 1 19 .72 23 .43 28 .99 31.70 2 22 .58 24 .83 32.01 32 .43 4 23 .03 25.72 33 .27 32.70 6 24 .00 26.01 35 .37 33 .36 8 24.17 27 .88 35 .58 37 .19

Examination of the table shows that as the carbonate content increases the expansion obtainable becomes greater, the temperature at which appreciable expansion takes plate becomes lower, and the time required to give a similar expansion at the same temperature becomes short- The highest heating temperature (300 C.) was lower than the melting point of lead (about 327 C.). If the TABLE II Elongation, percent 011 Volume increase, percent 1.6 ins. gauge length Tensile strength, lbs./

What we claim is: 1. A method of producing low density lead comprising the steps of obtaining a mixture of lead powder and a melting point had been exceeded the liberated gas would substance capable of liberating gas when heated to a temperature below the melting point of lead, working the mixture so that contiguous lead particles become pressure welded together to form a coherent mass with said substance dispersed therein, and thereafter heating said mass to a temperature below the melting point of lead but above that at which gas is liberated from said substance to liberate gas from said substance to form gas-filled cells in said mass.

2. A method according to claim 1, wherein the working of said mixture is produced essentially by the extrusion thereof.

3. A method of producing low density lead as claimed in claim 1 comprising the steps of obtaining lead powder with the particles thereof coated with a substance capable of liberating gas when heated to a temperature below the melting point of lead, working the powder to deform. the particles so that the coating substance breaks up into fragments and contiguous particles become pressure welded together to form a coherent mass with said fragments dispersed therein, and thereafter heating said mass to a temperature below the melting point of lead but above that at which gas is liberated from the coating substance.

4. A method according to claim 3, wherein the substance with which the particles of the lead powder are coated comp-rises lead carbonate.

5. A method according to claim 4 wherein the lead powder particles are so coated by exposing them to an atmosphere of carbon dioxide and water vapor.

6. A method according to claim 3, wherein the substance with which the particles of the lead powder are coated includes lead oxide and lead carbonate.

7. A method according to claim 6, wherein the lead powder particles are so coated by exposing them to air.

8. A method according to claim 6, wherein the lead powder particles are so coated by exposing them to an oxidizing atmosphere and to an atmosphere of carbon dioxide and water vapor.

9. A method of producing low density lead as claimed in claim 1 comprising the steps of obtaining a powder comprising lead particles and particles of a substance capable of liberating gas when heated to a temperature below the melting point of lead, working this powder so that contiguous lead particles become pressure welded together to form a coherent mass with the particles of said substance dispersed therein, and thereafter heating said mass below the melting point of lead but above that at which gas is liberated by said substance.

10. A method according to claim 9, wherein the gas liberating substance is lead carbonate powder admixed With the lead particles.

References Cited by the Examiner UNITED STATES PATENTS 1,919,730 7/1933 Koenig et a1. 222 2,935,396 5/1960 Pashak 75-222 2,937,938 5/1960 Fiedler et al. 75-222 X 2,974,034 3/1961 Fiedler et a1 75-222 X 2,979,392 4/1961 Fiedrler et a1 75222 X 3,216,824 11/1965 Boghen et al. 75212 X L. DEWAYNE RUTLEDGE, Primary Examiner,

R. L. GRUDZIECKI, Assistant Examiner. 

1. A METHOD OF PRODUCING LOW DENSITY LEAD COMPRISING THE STEPS OF OBTAINING A MIXTURE OF LEAD POWDER AND A SUBSTANCE CAPABLE OF LIBERATING GAS WHEN HEATED TO A TEMPERATURE BELOW THE MELTING POINT OF LEAD, WORKING THE MIXTURE SO THAT CONTIGUOUS LEAD PARTICLES BECOMES PRESSURE WELDED TOGETHER TO FORM A COHERENT MASS WITH SAID SUBSTANCE DISPERSED THEREIN, AND THEREAFTER HEATING SAID MASS TO A TEMPERATURE BELOW THE MELTING POINT OF LEAD BUT ABOVE THAT AT WHICH GAS IS LIBERATED FROM SAID SUBSTANCE TO LIBERATE GAS FROM SAID SUBSTANCE TO FORM GAS-FILLED CELLS IN SAID MASS. 